1. Field of the Invention
The present invention relates to a deflection yoke, and more particularly, to a deflection yoke able to improve a deflection sensitivity without affecting other characteristics of a cathode ray tube.
2. Description of the Related Art
Generally, a deflection yoke used in a cathode ray tube (CRT) of a television or a monitor is one of various yoke types, such as a saddle-toroidal type and a saddle-saddle type.
FIG. 1 is a cross-sectional view of a conventional deflection yoke 1. The deflection yoke 1 is symmetrical and is provided with a coil separator 10 having a pair of portions formed in an integrated body.
The coil separators 10 includes a screen unit 11 corresponding to a screen 2a of the CRT, a middle unit 12 extended from the screen unit toward a rear side of the CRT, and a neck unit 13 formed with the middle unit 12 in a body and coupled to an electron gun of the CRT.
On an inside and an outside of the coil separator 10 are provided a horizontal deflection coil 14 having a pair of upper and lower portions generating a horizontal deflection magnetic field to deflect en electron beam in a horizontal direction and a vertical deflection coil 15 having a pair of left and right portions generating a vertical deflection magnetic field to deflect the electron beam in a vertical direction, respectively. The horizontal deflection coil 14 and the vertical deflection coil 15 are collectively called a deflection coil. A ferrite core 16 is provided on an outside of the vertical deflection coil 15 to strengthen the horizontal deflection magnetic field and the vertical deflection magnetic field.
The horizontal deflection coil 14 is formed with front and rear ends having an outward flange shape and is also formed with a bend generating an unnecessary magnetic field.
The deflection yoke 1 has a horn shape having a gentle curve formed between a front part and a rear part of the coil separator 10.
The horizontal and vertical deflection coils 14, 15 have the same horn shape as the coil separator 10.
The vertical deflection coil 15 may be formed in a toroidal shape in which a coil is wound around the ferrite core 16, and the vertical deflection coil 15 having the toroidal shape in the deflection yoke 1 is called a saddle-saddle type.
FIG. 2 is a cross-sectional view of a coil separator 10 of the conventional deflection yoke 1 shown in FIG. 1. In the coil separator 20 of the conventional deflection yoke 1, a rear area of the middle area 22 and the neck area 23 have the same diameter so that a horizontal surface H is formed on a rear area of the coil separator 20 in an axial direction of the CRT.
The middle area includes a horn shaped area 22a having a horn shaped curve, and a linear area formed in a linear line in the axial direction of the CRT.
FIG. 3 is a perspective view of a horizontal deflection coil 30 of the deflection yoke shown in FIG. 1, FIG. 4 is a diagram showing the horizontal deflection coil 30 shown in FIG. 3, FIG. 5 is a side view of the horizontal deflection coil shown in FIG. 3, and FIG. 6 is a cross-sectional view taken along a-a′ and b-b′ of FIG. 5.
The horizontal deflection coil 30 as shown in FIGS. 3 through 6, is a non-bent type without the outward flange shape. Since a vertical deflection coil and the horizontal deflection 30 are the same in structure, an explanation of the vertical deflection coil will be omitted.
The horizontal deflection coil 30 includes a screen bent portion 31, an extension portion 32, and a neck bent portion 33 corresponding to the screen area 21, the middle area 22, and the neck area 30 of the coil separator 20, respectively.
The extension portion 32 is formed on a rear side of the screen bent portion 31, and the neck bent portion 33 is formed on a rear side of the extension portion 32 to form a single body with the extension portion 32.
The extension portion 32 includes a horn shaped portion 32a having a horn shaped curve, and a linear portion having a straight line in the axial direction of the CRT. The extension portion 32 generates the horizontal deflection magnetic field for deflecting the electron beam in the horizontal direction. The neck bent portion 33 generates an unnecessary magnetic field which does not contribute to generating of the horizontal deflection magnetic field, and is called a non-effective bent area.
As shown in FIGS. 4 through 6, an inside radius (DSO, DL) and an outside radius (FSO, FL) are formed in the horizontal deflection coil 30 from the horn shaped portion 32a to a rear portion of the neck bent portion 33. DSO and DL represent the inside radius in the vertical direction and the horizontal direction, respectively, and FSO and FL represent the outside radius in the vertical direction and the horizontal direction, respectively.
Since the unnecessary magnetic field is generated in the neck bent portion 33, a beam strike neck (BSN) distance is shortened due to the unnecessary magnetic field.
The BSN distance is a movement distance of a deflection point at which the electron beam starts to be deflected by the horizontal or vertical deflection magnetic field toward a predetermined position of a screen of the screen unit 11, and the BSN distance is disposed to be shifted toward the electron gun by the movement distance since the deflection yoke 1 is not closely attached to a rearmost portion of the CRT but is installed to be spaced-apart from the rearmost portion of the CRT according to the movement distance.
The movement distance of the BSN is called the BSN distance. If the BSN distance is lengthened, the electron beam is able to reach an outermost peripheral portion of the screen of the CRT due to a maximum deflection of the electron beam. However, if the BSN distance is shortened, the electron beam is not able to reach the outermost peripheral portion of the screen of the CRT but strikes an inner surface of the CRT. As a result, a dark area is shown in a corner of the screen, and it is impossible to properly display a display image on the screen.
FIGS. 7A through 8 describe a state that the electron beam is influenced by the horizontal deflection magnetic field generated from the extension portion 32 of the horizontal deflection coil 30 and a magnetic field (another vertical deflection magnetic field) generated from the neck bent portion 33.
FIGS. 7A, 7B, and 7C are diagrams showing magnetic fields generated from the horizontal deflection coil 30 shown in FIG. 3, and FIG. 8 is a diagram showing a relationship between the electron beam and the magnetic field generated from a neck bent portion 33 of the horizontal deflection coil 30 shown in FIG. 3.
As shown in FIGS. 7A and 7B, the diagram of FIG. 7B which is taken along a line a-a′ of FIG. 7A, shows that a horizontal deflection force F1 is generated in a horizontal deflection magnetic field B1 of the extension portion 32 of the horizontal deflection coil 30, and the electron beam emitted from an electron gun 2b is deflected in an X direction.
Current I is defined by a reversed direction opposite to an emitting direction of the electron beam of the electron gun 2b, that is, an reversed direction of the electron beam. The horizontal deflection force F1 corresponds to a first magnetic force B1 generated by the current I flowing through the extension portion 32 as shown in FIG. 7A.
The diagram of FIG. 7C which is taken along a line b—b of FIG. 7A, describes that a vertical deflection magnetic field B2 is generated from the neck bent portion 33 of the horizontal deflection coil 30 in a direction opposite to the electron beam due to the current flowing in the horizontal direction. As a result, a vertical deflection force F2 is generated in a Y direction toward a top portion of the neck bent portion 33. The vertical deflection force F2 corresponds to a second magnetic force B2 generated by the current I flowing through the neck bent portion 33 as shown in FIG. 7A.
The vertical deflection force F2, which is generated by the vertical deflection magnetic field B2 of the neck bent portion 33 of the horizontal deflection coil 30, does not strengthen the horizontal deflection force F1 deflecting the electron beam in the horizontal direction but weakens the horizontal force F1 by deflecting the electron beam a direction other than the X direction, thereby causing the BSN distance to be shortened.
The vertical deflection magnetic field B2 is formed in a fan shape in an outward radial direction due to a round shape of the neck bent portion 33. Since the vertical deflection magnetic field B2 is generated in a corner area of the neck bent portion 33 in a diagonal direction with respect to the direction of the electron beam, the electron beam is influenced by the vertical deflection magnetic field B2 as shown in FIG. 8.
R, G, B electron beams GB are not formed in a single spot on the screen but are formed on a line, and a gap is formed between the adjacent R, G, B electron beams as shown in FIG. 8.
The R, G, and B electron beams are focused toward the single spot, and the R and B electron beams GB are inclined with respect to the G electron beam.
The current I is defined in the reversed direction opposite to the emitting direction of the electron beam emitted from the electron gun 2b, and an inclined angle θ is formed between the reversed direction of the current I and the vertical deflection magnetic field B2 formed in a direction opposite to the emitting direction of the electron beam.
Since the inclined angle θ corresponds to sin θ according to vector F=IB sin θ, a deflection force F deflecting the electron beam is generated in the neck bent portion 33 of the horizontal deflection coil 30 Since the inclined angle θ becomes greater in the vertical deflection magnetic field B2 generated in the diagonal direction in the corner area of the neck bent portion 33, the electron beam disposed in an area corresponding to the corner area of the neck bent portion 33 is deflected greater than other electron beam disposed in other area.
However, the vertical deflection magnetic field B2 is not significant compared to the horizontal deflection magnetic field B1 generated from the horizontal deflection coil 30. Therefore, the vertical deflection magnetic field B2 does not affect a horizontal deflection of the electron beam. However, a problem that the BSN distance is shortened due to the vertical deflection magnetic field B2 occurs.
FIG. 9 is a diagram showing a scanning state of the electron beam according to a current of the horizontal deflection coil 30 of the conventional deflection yoke shown in FIG. 1. The scanning state shows a state of the electron beam scanned according to the current I flowing through the horizontal deflection coil 30.
The electron beam horizontally deflected by the horizontal deflection coil 30 is horizontally scanned on the screen 2a of the CRT according to a saw-type current IR, IL flowing through the horizontal deflection coil 30.
The electron beam is horizontally deflected toward a rightmost side of the screen 2a in response to a maximum current IR and toward a leftmost side of the screen 2a of the screen unit 11 in response to a minimum current IL. This correlation between a scanning width and respective magnitude of the current IR, IL corresponds to a deflection sensitivity.
The deflection sensitivity is represented by a formula of a product of an inductance L (inductance of the horizontal deflection coil 30) and a square I2 of the current I flowing through the horizontal deflection coil 30).horizontal deflection sensitivity mHA2=I2×L  FORMULA
That is, the deflection sensitivity of the horizontal deflection coil 30 is a product of the inductance L and a square of the maximum current IR and/or the minimum current IL
An efficiency of the CRT is improved when a consumed current is small during deflecting the electron beam to the rightmost side and the leftmost side of the screen 2a. Accordingly, the defection sensitivity is improved when a value of the deflection sensitivity is small.
The deflection sensitivity is defined by respective final values of the inductance L and the current I consumed when the electron beam reaches the rightmost side and the leftmost side of the screen 2a. 
In order to improve the deflection sensitivity, the inside radius DL, DSO and the outside radius FL, FSO of the horizontal deflection coil 30 should be shortened. However, there is a limitation in minimizing the inside radius DL, DSO and the outside radius FL, FSO of the horizontal deflection coil 30 since the inside radius DL, DSO and the outside radius FL, FSO of the horizontal deflection coil 30 should be larger than a diameter of the electron gun 2b when the deflection yoke 1 is mounted on the CRT.
FIG. 10 is a diagram showing the deflection sensitivity and a BSN phenomenon of the horizontal deflection coil 30 of the conventional deflection yoke 1 shown in FIG. 1. As shown in FIG. 10, the BSN phenomenon occurs when the deflection yoke 1 is not disposed close to a rear side of the CRT but moves toward the electron gun 2b, and the electron beam is not able to reach a maximum point of the screen 2a which is disposed in one of the corner area, the rightmost side, and the leftmost side of the screen 2a, thereby generating a dark image in the corner area of the screen 2a since the electron beam strikes the rear side of the CRT as indicated in a broken line GB3 of FIG. 10.
The deflection yoke 1 first closely sticks to the rear side of the CRT and then moves backward toward the electron gun 2b during adjusting a convergence of the CRT. The maximum point is the corner area disposed in uppermost and lowermost sides of the screen 2a. 
When the deflection yoke 1 moves backward, an adjustment degree of the deflection yoke 1 is improved, and a manufacturing efficiency of the CRT is improved since the deflection yoke 1 is moved in one of upper, lower, horizontal, and vertical directions to adjust the convergence of the CRT.
If a backward moving distance of the deflection yoke 1 toward the electron gun 2b is lengthened, the adjustment degree of the deflection yoke 1 becomes improved. It is necessary to obtain a maximum value of the backward moving distance of the deflection yoke 1 as long as the BSN phenomenon is prevented.
In the correlation between the deflection sensitivity and the BSN phenomenon, the deflection sensitivity is inverse proportional to the BSN phenomenon. If the deflection sensitivity of the deflection yoke 1 is strengthened, an deflection angle of the electron beam increases since the magnetic field is strengthened, and the BSN distance is shortened. To the contrary, if the deflection sensitivity of the deflection yoke 1 is lowered, the deflection angle of the electron beam decreases since the magnetic field is weakened, and the BSN distance is lengthened.
That is, the BSN phenomenon becomes worsened according to the shortened BSN distance and the improved deflection sensitivity, and the BSN phenomenon becomes improved according to the lengthened BSN distance and the lowered deflection sensitivity.
When two different deflection yoke 1 having two different deflection sensitivities are closely attached to the rear side of the CRT, the electron beam is deflected as indicated as a beam path GBlwhen the deflection yoke 1 has the strengthened deflection sensitivity, and the electron beam is deflected as indicated as another beam path GB2 when the deflection yoke 1 has the lowered deflection sensitivity.
The beam path GB1 of the electron beam is moderately deflected compared to the consumed current of the deflection yoke 1 and is not able to reach the maximum point of the screen 2a, and the beam path GB2 of the electron beam is able to reach the maximum point of the screen 2a. 
The deflection efficiency of the deflection yoke 1 is not improved in accordance with the strengthened (worsened) deflection sensitivity of the deflection yoke 1 and is improved due to the lowered (improved) deflection sensitivity of the deflection yoke 1.
When the deflection coil 30 moves backward toward the electron gun 2b.of the CRT to the adjust the convergence, a deflection point moves to a position f′, and the BSN phenomenon that the electron beam strikes an inside surface of the electron gun 2b of the CRT occurs according to a movement of the deflection point as indicated as GB3 of FIG. 10.
The BSN distance is represented by a movement of the deflection point, i.e., a moving distance of the deflection yoke 1, and is defined by a distance between the positions f and f′ as shown in FIG. 10.
Therefore, the beam path GB1 of the electron beam in the CRT having the strengthened deflection sensitivity shows that the deflection point of the BSN phenomenon (BSN distance) is shortened according to the distance between the positions f and f′. The beam path GB2 of the electron beam in the CRT having the lowered deflection sensitivity shows that the deflection point of the BSN phenomenon (BSN distance) is lengthened according to the distance between the positions f and f′.
The deflection sensitivity may be too much strengthened (worsened) in favor of an increase of the BSN distance. However, the CRT should be designed to have the improved (lowered) deflection sensitivity rather than the increase of the BSN distance since the deflection sensitivity is a primary concern and a major factor in designing the CRT.
Accordingly, it is desirable to design the deflection yoke 1 having the improved (lowered) deflection sensitivity and the lengthened BSN distance.
The vertical deflection magnetic field B2 generated from the neck bent portion 33 of the horizontal deflection coil 30 deflects the electron beam in the vertical direction and causes the BSN distance to be shortened. Particularly, the vertical deflection magnetic field B2 having a component generated from a corner portion of the neck bent portion 33 in the diagonal direction cause the BSN distance to be more shortened due to the enlarged inclined angle θ.
As described above, it is disadvantageous in the deflection yoke 1 of the conventional CRT to shorten the BSN distance although the deflection sensitivity is slightly improved because the deflection yoke 1 is first closely attached to the rear side of the CRT and then moves backward toward the electron gun 2b. 
When the BSN distance is lengthened when the deflection yoke 1 moves backward toward the electron gun 2b, the deflection sensitivity decreases due to an increase of a value of the deflection sensitivity.
According to a movement of the deflection yoke 1 in the backward direction toward the electron gun 2b, the BSN distance is shortened, and the electron beam is not able to reach the maximum point of the screen 2a but strikes an inside surface of the rear side of the CRT, thereby causing a portion of a display image not to be displayed on the screen 2a. 